Filamentation and damage in fused silica induced by tightly focused femtosecond laser pulses

We investigate experimentally and numerically the damage tracks induced by tightly focused $(\mathrm{NA}=0.5)$ infrared femtosecond laser pulses in the bulk of a fused silica sample. Two types of irreversible damage are observed. The first damage corresponds to a permanent change of refractive index without structural modifications (type I). It appears for input pulse energies beyond $0.1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{J}$. It takes the form of a narrow track extending over more than $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ at higher input powers. It is attributed to a change of the polarizability of the medium, following a filamentary propagation which generates an electron-hole plasma through optical field ionization. A second type of damage occurs for input pulse energies beyond $0.3\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{J}$ (type II). It takes the form of a pear-shaped structural damage associated with an electron-ion plasma triggered by avalanche. The temporal evolution of plasma absorption is studied by pump-probe experiments. For type I damage, a fast electron-hole recombination is observed. Type II damage is linked with a longer absorption.